1. Field of the Invention
This invention relates generally to separation of gaseous mixtures by selective adsorption and is more particularly concerned with a pressure swing adsorption system designed and operated for separate recovery from a multicomponent gas mixture of a primary key component and a secondary key component, each substantially freed of other dilute components present in the original gas mixture subject to treatment. To effect such separate recovery of desired components from the feed gas mixture the system of the invention uses separate beds of adsorbent in series gas flow therebetween during the adsorption stage, yet designed to be regenerated separately, using different regeneration or desorption procedures. Most particularly, the invention is chiefly concerned with the separate recovery of hydrogen and methane from gas mixtures including these components in addition to C.sub.2 and higher hydrocarbons.
2. Prior Art
Pressure swing cyclic adsorption systems designed for fractionation of gaseous mixtures by selective adsorption are well known in the art. In these systems one or more desired components of the feed gas mixture are separately recovered at a yield and purity depending upon the efficiency of the designed operation.
Illustrative of typical systems indicated to be especially useful in the recovery of hydrogen from gaseous mixtures with CH.sub.4 and/or CO.sub.2 are those described in U.S. Pat. Nos. 3,138,439; 3,142,547; 3,788,037. Other patents describe in general systems for separation of essentially binary gas mixtures or of multicomponent gas mixtures, asserted to be applicable in recovery of hydrogen from such mixtures. Illustrative of these are the systems described, for example, in U.S. Pat. Nos. 3,221,476; 3,430,418 and 3,720,042. Also among the systems described in the prior patent art are those employing separate adsorbent beds operated in series flow and designed for, or stated to be applicable in, separate recovery of hydrogen and one or more other components present in a multicomponent feed gas mixture. Typical among such systems are those described in U.S. Pat. Nos. 3,102,013; 3,149,934; 3,176,444; 3,237,379; 3,944,400 and 4,000,990.
Among the various prior art patents relating to pressure swing adsorption, the recovery of hydrogen from multicomponent gas streams also containing methane and higher hydrocarbons is particularly disclosed in U.S. Pat. Nos. 3,102,013; 3,142,547 and 3,176,444.
In U.S. Pat. No. 4,077,779, there are described adsorption systems designed primarily for separation of binary gas mixtures which may contain trace amounts of other impurities. While the systems therein described can be satisfactorily operated in separation of such binary gas mixtures, these systems cannot be efficiently utilized in the treatment of multicomponent gas mixtures containing in addition to two major key components a certain quantity (more than trace amounts) of one or more dilute contaminants. The presence of such dilute components adversely affects the efficiency of gas separation by pressure swing adsorption techniques designed for handling essentially binary gas mixtures.
In typical multicomponent gas mixtures confronted in industry for desired separation and recovery therefrom of desired components there are generally present two bulk key components accompanied by a certain quantity of dilute impurities. One of the key components, generally present in predominant quantity, consitutes the desired primary product, which can be separated and recovered at high purity by pressure swing adsorption. The remaining bulk component is recoverable as a secondary product. The other dilute constituent gases present in said multicomponent gas mixtures are usually of such nature and/or present in such amount that there is little incentive for their separate recovery in enriched form. An impure stream of these dilute components or their presence in the recovered secondary product is often acceptable.
Multicomponent gas mixtures generally encountered in industrial gas separation can be classified into two different groups:
(1) Such mixtures in which the minor dilute components are less strongly adsorbed than the secondary key component by the adsorbent chosen for the process.
(2) Such mixtures in which the minor diluent components are more strongly adsorbed than the secondary key component.
An example of a mixture of the first type is the gaseous effluent from a shift converter in a hydrocarbon reforming plant. A typical composition of such effluent may be 76% H.sub.2, 20% CO.sub.2, 3.5% CH.sub.4 and 0.5% CO (each by volume). From such mixture CO.sub.2 is to be removed and hydrogen recovered as primary component in substantially pure state. The dilute impurities, CO and CH.sub.4 are less strongly sorbed than CO.sub.2 on most commercial sorbents such as activated carbon and molecular sieves.
An example of a mixture of the second type is the effluent gas from a hydrodesulfurization plant, wherein it is desired to remove CH.sub.4 to recover high purity hydrogen for recycle. A typical hydrodesulfurization plant effluent gas after removal of the sulfur compounds may contain 65% H.sub.2, 20% CH.sub.4, and 5% each of C.sub.2, C.sub.3 and C.sub.4 -C.sub.6 hydrocarbon components (each by volume). In this instance also, hydrogen constitutes the primary component. The dilute C.sub.2 + hydrocarbons are usually more strongly sorbed than the secondary component (CH.sub.4) on most commercial sorbents.
The present invention is particularly concerned with the separation of multicomponent gas mixtures of the second type hereinabove described. The primary requirement for separation of such gas mixtures is that the hydrogen be recovered at high purity and at high yield so that it can be recycled efficiently to the desulfurization plant.
This requirement can be successfully met by practice of the present invention. In addition, the present invention provides an opportunity to produce a secondary product stream containing substantially pure CH.sub.4 along with a tertiary product stream containing the C.sub.2 + hydrocarbons, some CH.sub.4 and some H.sub.2. This offers a choice to the user who may prefer to utilize the CH.sub.4 byproduct stream elsewhere instead of using it as fuel for economic reasons. The tertiary product stream can be used as a fuel gas.